revert/rework 81647a9a('fix load_ioengine() not to support no "external:" prefix')

revert/rework 81647a9a('fix load_ioengine() not to support no "external:" prefix')

We have decided to revert 81647a9a, which disallowed loading an external
ioengine without using (explicitly documented)"external:" prefix.

[PATCH 4/5] fix load_ioengine() not to support no "external:" prefix
http://www.spinics.net/lists/fio/msg06241.html

> The current implementation (i.e. do 2 if 1 failed) happened to have
> been able to load an external ioengine with below syntax without
> "external:" prefix, but this is a bug rather than a feature.

This commit reverts what 81647a9a does, while keeping a commit made
on top of it. See below for details, which is also available in above
mailing list thread. Lines marked with \ are comments by Jens.

--
  >>>>>>> People have already started depending on the "wrong" behaviour e.g.
  >>>>>>> ioengine=libfio_ceph_objectstore.so # must be found in your LD_LIBRARY_PATH
  >>>>>>> from https://github.com/ceph/ceph/blob/master/src/test/fio/ceph-bluestore.fio
  >>>>>>> . Because they're external it's hard to find out just how prevalent
  >>>>>>> this behaviour is.
  >>>>>>
  >>>>>> Then they can add "external:".
  >>>>>> If it has ever been official (in docs/etc), then the program should
  >>>>>> maintain such behavior, like fio keeps kb_base=1024 by default.
  >>>>>>
  >>>>>> This isn't the first time fio broke external ioengines to begin with,
  >>>>>> and fio doesn't guarantee external ioengines.
  >>>>>> e.g. this commit more than a year ago broke ABI.
  >>>>>> 565e784df05c2529479eed8a38701a33b01894bd
  >>>>>>
  >>>>>> It's essentially the same as non mainline'd kernel modules are on
  >>>>>> their own to be uptodate with upstream.
  >>>>>
 \>>>>> No, those two are VERY different. Fio doesn't make any guarantees wrt
 \>>>>> keeping the same ABI (or API, even) for external engines. If they break,
 \>>>>> then you get to keep the pieces and glue them back together. We have
 \>>>>> the engine version number as well to guard against those kinds of
 \>>>>> changes.
 \>>>>>
 \>>>>> But we don't break job files, at least not knowingly, and definitely
 \>>>>> not just in the name of a cleanup.
  >>>>
  >>>> Yes, but you took this patch which breaks job files *wrongly* using
  >>>> external engine option syntax.
  >>>
 \>>> Right, but it's not in a release yet, so it's not impossible to just
 \>>> revert it.
  >>>
  >>>> By "job files wrongly using", I mean job files somehow found
  >>>> unofficial (i.e. undocumented, not intended) way to dlopen it.
  >>>
 \>>> This case is a bit difficult, since it isn't black and white. I took a
 \>>> look at some of the older documentation, and we have been documenting
 \>>> the need for 'external' for a long time (forever?). So I'd be inclined
 \>>> to let this one stay in.
  >>>
  >>>> If breaking this case is not okay, shouldn't this commit be reverted
  >>>> or the design be expand to allow both with/without prefix ?
  >>>
 \>>> The design (or intent, at least) has always been that if you do:
 \>>>
 \>>> ioengine=something
 \>>>
 \>>> then we first try and load 'something' as an internal engine. If that
 \>>> fails, we look for an external engine of that name. That makes sense to
 \>>> me, since it means you don't have to do anything special to load an
 \>>> external engine. I think we should try and retain that behavior, if we
 \>>> can.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>